Semiconductor structure and operating method for improving charge transfer of image sensor device

ABSTRACT

An image sensor semiconductor device includes a semiconductor substrate and a first photodiode disposed in the semiconductor substrate and configured to generate charges in response to radiation. The image sensor semiconductor device also includes a first transistor disposed adjacent to the first photodiode, and a second transistor disposed over the first photodiode, wherein the first transistor and the second transistor are configured to generate at least one electric field to move the charges generated by the first photodiode. The image sensor device further includes a floating diffusion region configured to store the moved charges.

BACKGROUND

As technologies evolve, complementary metal-oxide semiconductor (CMOS)image sensors are attracting more attention due to performanceadvantages. For example, CMOS image sensors can provide higher imageacquisition rates, lower operating voltages, lower power consumption andgreater noise immunity. A CMOS image sensor usually comprises an arrayof light-sensing elements or pixels. Each of the pixels is configured toconvert received photons into electrons. Additionally, the CMOS imagesensor comprises circuitry to transform the electrons into electricalsignals. The electrical signals are then processed to generate an imageof a subject scene.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1A and 1B are a schematic cross-sectional view and a schematicplan view, respectively, of an image sensor device, in accordance withsome embodiments.

FIGS. 1C and 1D are schematic layouts of doped layers of the imagesensor device in FIG. 1A, in accordance with some embodiments.

FIG. 2 is a schematic circuit diagram of an image sensor device, inaccordance with some embodiments.

FIGS. 3A and 3B are schematic diagrams of potential profiles associatedwith the image sensor device of FIG. 1A, in accordance with someembodiments.

FIG. 4A is a timing diagram of a readout operation of an image sensordevice, in accordance with some embodiments.

FIG. 4B is a schematic potential diagram associated with the readoutoperation in FIG. 4A, in accordance with some embodiments.

FIGS. 4C through 4F are schematic potential diagrams associated with thereadout operation in FIG. 4A, in accordance with some embodiments.

FIG. 5A is a timing diagram of a shutter operation of an image sensordevice, in accordance with some embodiments.

FIGS. 5B, 5C and 5D are schematic potential diagrams associated with theshutter operation in FIG. 5A, in accordance with some embodiments.

FIGS. 6A and 6B are a schematic circuit diagram and a plan view,respectively, of an image sensor device, in accordance with someembodiments.

FIGS. 6C and 6D are schematic layouts of doped layers of the imagesensor device in FIG. 6A, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath.” “below,” “lower,”“above.” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The present disclosure provides a semiconductor image sensor device andoperating methods thereof according to various embodiments. Theperformance of an image sensor is determined by several factors and theblooming effect is one of them. The maximal amount of charge of acertain photodiode in an image sensor that can be collected and detectedis a fixed value. As charges are generated and collected in a photodiodeof the image sensor, it may reach the photodiode well capacity. If theamount of charges exceeds the well capacity, some charges may flow outof their original pixel and enter adjacent pixels, a phenomenon referredto as blooming. The blooming effect will induce noise to the generatedimage, thus deteriorating image quality. The blooming effect may be morepronounced in a partially-isolated image sensor where isolationstructures formed between adjacent pixels only partially demarcatepixels. Moreover, existing charge-transferring transistors may not movethe generated charges efficiently and may introduce image lagaccordingly. In the present disclosure, an auxiliary transistor andoperating methods thereof are introduced to improve the performance ofpartially-isolated image sensors. Residual photocharge left in thephotodiode not fully cleaned by existing charge-transferring circuitsmay be further drained by help of the anti-blooming transistor duringthe shutter phase. Additionally, the auxiliary transistor is furtherconfigured to amplify the electric field in the readout phase so as tofacilitate charge transfer. As a result, the blooming effect and theimage lag can be reduced.

FIGS. 1A and 1B are a schematic cross-sectional view and a plan view,respectively, of an image sensor device 100, in accordance with someembodiments. The cross-sectional view shown in FIG. 1A is taken along asectional line AA′ in FIG. 1B. The image sensor device 100 may be acomplementary metal-oxide-semiconductor (CMOS) image sensor or an activepixel sensor. In some embodiments, the image sensor device 100 is acharge-coupled device (CCD) or a passive pixel sensor. In someembodiments, the image sensor device 100 is a back-side illuminated(BSI) image sensor. Referring to FIG. 1A, the image sensor device 100includes a substrate 101, a first photodiode 102, a second photodiode104, a floating diffusion region 126, a transfer transistor 130, anauxiliary transistor 140, a reset transistor 150, an amplifiertransistor 160, and a readout transistor 170. In some embodiments, theimage sensor device 100 includes a unit pixel as viewed from above,wherein the unit pixel may include a vertical photodiode stack that atleast comprises the first photodiode 102 and the second photodiode 104.In the depicted embodiment, only one unit pixel is illustrated. However,it is understood that an array of unit pixels arranged in rows andcolumns are within the contemplated scope of the present disclosure. Insome embodiments, a unit pixel further includes the floating diffusionregion 126 and a signal processing circuit including at least one of thetransistors 130, 150, 160 and 170.

The substrate 101 includes a semiconductor material such as silicon orgermanium. In some embodiments, the substrate 101 may include othersemiconductor materials, such as silicon germanium, silicon carbide,gallium arsenide, gallium phosphide, indium phosphide, indium arsenide,indium antimonide, or combinations thereof. The substrate 101 may bedoped with an N-type dopant, such as arsenic or phosphor, and may bedoped with a P-type dopant, such as boron or the like. The substrate 101has a first surface 101A (sometimes termed as a back side) and a secondsurface 101B (sometimes termed as a front side) opposite to the firstsurface 101A. In some embodiments, the image sensor device 100 isconfigured to receive radiation incident from the first surface 101A forthe BSI image sensor. In some embodiments, active components, such astransistors, or passive components, such as doped regions, conductivefeatures or dielectric layers, are formed on the second surface 101B.

The first photodiode 102 and the second photodiode 104 are formed in thesubstrate 101, wherein the first photodiode 102 is comprised of a firstlayer 102P and a second layer 102N, and the second photodiode 104 iscomprised of a first layer 104P and a second layer 104N. In someembodiments, the first photodiode 102 and the second photodiode 104 arestacked along a depth direction of the substrate 101 so that the pixelwell capacity can be magnified while the pixel footprint is minimized. Ajunction in each of the photodiodes 102 and 104 is formed between therespective first layer (102P/104P) and second layer (102N/104N), and acorresponding depletion region is established accordingly. The receivedradiation or photons are transformed into charges around the depletionregions of the respective photodiode. In the depicted embodiment, thesubstrate 101 has a P-type dopant. The first layers 102P and 104P of therespective photodiodes 102 and 104 may be doped layers having a P-typedopant. Moreover, the second layers 102N and 104N may be doped layershaving an N-type dopant.

In some embodiments, the doping concentration of the first layers 102Pand 104P is greater than the doping concentration of the substrate 101.In some embodiments, the first layers 102P and 104P have a dopingconcentration between about 1E17 atoms/cm³ and about 1E19 atoms/cm³. Insome embodiments, the first layers 102P and 104P have a dopingconcentration between about 1E17 atoms/cm³ and about 5E18 atoms/cm³. Insome embodiments, the first layers 102P and 104P have a dopingconcentration between about 5E17 atoms/cm³ and about 5E18 atoms/cm³. Insome embodiments, the second layer 102N of the first photodiode 102 hasa doping concentration between about 1E17 atoms/cm³ and about 1E19atoms/cm³. In some embodiments, the second layer 102N of the firstphotodiode 102 has a doping concentration between about 1E17 atoms/cm³and about 5E18 atoms/cm³. In some embodiments, the second layer 102N ofthe first photodiode 102 has a doping concentration between about 5E17atoms/cm³ and about 5E18 atoms/cm³. In some embodiments, the secondlayer 104N of the second photodiode 105 has a doping concentrationbetween about 5E16 atoms/cm³ and about 5E17 atoms/cm³. In someembodiments, the second layer 104N of the second photodiode 105 has adoping concentration between about 1E17 atoms/cm³ and about 5E17atoms/cm³.

An isolation structure 112 is formed in the substrate 101 for isolatinga unit pixel of the image sensor device 100 from adjacent unit pixels.In some embodiments, the isolation structure 112 is exposed on the firstsurface 101A of the substrate 101. In some embodiments, the isolationstructure 112 laterally surrounds the second layer 104N of the secondphotodiode 104. In some embodiments, the isolation structure 112 doesnot penetrate through the substrate 101; therefore, adjacent unit pixels(or photodiodes) are not fully separated. Such partial isolationstructure 112 is so arranged as to be compatible with other featuresformed on the second surface 101B of the substrate 101. In someembodiments, the isolation structure 112 has an upper surfacesubstantially level with the second layer 104N of the second photodiode104. The isolation structure 112 may be formed of a doped semiconductormaterial, a dielectric material, or combinations thereof. The dopedsemiconductor material for the isolation structure 112 may be a P-typedoped silicon material with a doping concentration. The dielectricmaterial may include an oxide, a nitride, an oxynitride, silicondioxide, a polymer material, or the like.

A transfer transistor 130 is formed on the second surface 101B. In someembodiments, the transfer transistor 130 includes a gate electrode 130Gand a gate dielectric 132. The gate electrode 130G is configured toestablish an electric field that moves photocharges in the photodiodes102 and 104 to the floating diffusion region 126. The gate electrode130G is formed of a conductive material, such as a metallic material(e.g., tungsten, copper, silver) or polysilicon. The gate electrode 130Gmay include a vertical portion 130V penetrating into the substrate 101in a depth direction and a horizontal portion 130H extending over thefront surface 101B of the substrate 101. The gate dielectric 132 isdisposed between the gate electrode 130G and the substrate 101. The gatedielectric 132 may include silicon oxide, silicon nitride, siliconoxynitride, other suitable materials, or combinations thereof. The gatedielectric may include high-k materials such as metal oxides, metalnitrides, metal silicates, transition metal-oxides, transitionmetal-nitrides, transition metal-silicates, oxynitride of metals, metalaluminates, zirconium silicate, zirconium aluminate, hafnium oxide, orcombinations thereof.

The horizontal portion 130H may have a first protrusion having adistance D1 measured from a first sidewall of the vertical portion 130Vto a first side 130L and a second protrusion having a distance D2measured from a second sidewall of the vertical portion 130V to a secondside 130R. In some embodiments, the distances D1 and D2 are not equal.In some embodiments, the distance D1 is made greater than the distanceD2 to ensure better coverage of the electric field generated by the gateelectrode 130G.

The floating diffusion region 126 is disposed adjacent to the transfertransistor 130. In some embodiments, the floating diffusion region 126is disposed on a side of the transfer transistor 130 opposite the firstphotodiode 102. The floating diffusion region 126 acts as a charge tankin which charges can be read out in a readout operation. In the depictedembodiment, the floating diffusion region 126 has an N-type dopant. Insome embodiments, the floating diffusion region 126 has a dopingconcentration greater than a doping concentration of the first layer102N or 104N.

A semiconductor region 103 is formed in the substrate 101 between thegate dielectric 132 and the first photodiode 102 or the secondphotodiode 104. The semiconductor region 103 surrounds a sidewall and abottom of the vertical portion 130V. In some embodiments, thesemiconductor region 103 contacts an edge of the second layer 102N ofthe first photodiode 102. In some embodiments, the semiconductor region103 contacts an upper surface of the second layer 104N of the secondphotodiode 104. When a recess is formed in which the gate electrode 130Gis to be subsequently deposited, lattice defects may be introducedaround the interface of the substrate 101 and layers lined to therecess, such as the gate dielectric 132. Noise such as dark current maybe generated due to such defects. As a remedy, the semiconductor region103 is formed at the interface so as to remove charges (electrons) fromthe dark current. In the depicted embodiment, the semiconductor region103 may be formed of a doped P-type region with a doping concentrationgreater than the doping concentration of the substrate 101.

The auxiliary transistor 140 is formed on the second surface 101B of thesubstrate 101. The auxiliary transistor 140 is disposed adjacent to thefirst photodiode 102. The auxiliary transistor 140 is introduced toenhance device performance during the shutter phase and the readoutphase, operation details of which will be described in subsequentparagraphs. The auxiliary transistor 140 includes a gate electrode 140G,a gate dielectric 144 and a source/drain region 142. Charges may bedrained to the source/drain region 142 during a shutter operation viacontrol of the gate electrode 140G. The gate electrode 140G has amaterial and a configuration similar to those of the gate electrode 130Gof the transfer transistor 130. The gate dielectric 144 has a materialand a configuration similar to those used in the gate dielectric 132. Insome embodiments, the source/drain region 142 has an N-type dopant.

The first photodiode 102 may be disposed closer to the second surface101B than the second photodiode 104, thus the first photodiode 102 andthe second photodiode 104 are sometimes termed as “shallow photodiode”(SPD) and “deep photodiode” (DPD), respectively. The first photodiode102 and the second photodiode 104 may have different arrangements. Thefirst layer 102P has an area less than an area of the second layer 102Nwherein both areas are viewed from above. Although not illustrated, thesecond layer 102N may have a shape substantially following the patternof the first layer 102P. In some embodiments, the first layer 102P andthe second layer 102N extend below the gate electrode 140G of theauxiliary transistor 140 where the second layer 102N extends fartherthan the first layer 102P. The first layer 102P overlaps the gateelectrode 140G by a width W1. In some embodiments, the width W1 isgreater than about 10 nm. The first layer 102P overlaps the gateelectrode 130G by a width W2. In some embodiments, the width W2 isgreater than about 10 nm. The first layer 102P is distant from asidewall of the vertical portion 130V by a width W3. In someembodiments, the width W3 is greater than about 20 nm. In someembodiments, the second layer 102N also extends below the horizontalportion 130H of the gate electrode 130G. The second layer 102N maycontact a sidewall of the semiconductor region 103 adjacent to thevertical portion 130V.

The first layer 104P of the second photodiode 104 extends below thesecond layer 102N of the first photodiode 102. The first layer 104P isformed on the isolation structure 112. In some embodiments, the firstlayer 104P partially overlaps the second layer 104N. In someembodiments, the first layer 104P covers a portion of the isolationstructure 112 for mitigating leakage current. In some embodiments, thefirst layer 104P contacts an upper surface of the isolation structure112. In some embodiments, the first layer 102P is separated from thegate electrode 130G of the transfer transistor 130. A channel 101Cformed of the substrate material 101 between the semiconductor region103 and the first layer 104P of the second photodiode 104 is created forfacilitating charge movements between the first photodiode 102 and thesecond photodiode 104. The blooming effect can be relieved due to thechannel 101C.

The reset transistor 150, the amplifier transistor 160 and the selecttransistor 170, herein collectively referred to as a readout circuit120, are formed on the second surface 101B of the substrate 101. Thereset transistor 150 includes a gate electrode 150G, a gate dielectric154, and two source/drain regions 156 and 158. The amplifier transistor160 includes a gate electrode 160G, a gate dielectric 164, and twosource/drain regions 166 and 168. In some embodiments, the amplifiertransistor 160 is configured as a source follower. The readouttransistor 170 includes a gate electrode 170G, a gate dielectric 174,and two source/drain regions 176 and 178, where one source/drain region176 may be shared with the source/drain region 168 of the amplifiertransistor 160. The gate electrodes 150G. 160G and 170G have materialsand configurations similar to those used in the gate electrode 130 or140. The gate dielectrics 154, 164 and 174 include materials andconfigurations similar to those used in the gate dielectric 132 or 144.

The reset transistor 150 is configured to preset the voltage at thefloating diffusion region 126 to a predetermined voltage, e.g., a supplyvoltage, during a shutter operation. The accumulated charges in thefloating diffusion region 126 will be drained through the resettransistor 150 accordingly. During a readout operation, the amplifiertransistor 160 and the select transistor 170 are configured to providean image data voltage at the source/drain region 178 of the readouttransistor 170 in response to the potential at the floating diffusionregion 126. In some embodiments, the floating diffusion region 126 iselectrically coupled to the source region 156 of the reset transistor150 and the gate electrode 160G of the amplifier transistor 160, througha conductive path 188, for performing shutter and data readoutoperations. In some embodiments, the conductive path 188 is a conductivetrace or an interconnect layer including conductive materials andinsulating materials. In some embodiments, the conductive path 188 isdisposed in the substrate 101 or external to the substrate 101.

In some embodiments, the transistors of the readout circuit 120 areformed in a well region 114. In some embodiments, the well region restson the first layer 104P of the second photodiode 104. In someembodiments, the well region 114 abuts the second layer 102N of thefirst photodiode 102. In the depicted embodiment, the well region 114 isformed of a P-type doped region, but it can also be an N-type dopedregion in alternative embodiments. In some embodiments, the well region114 has a doping concentration greater than the doping concentration ofthe substrate 101. In some embodiments, the well region 114 has a dopingconcentration less than the doping concentration of the first layer 102Por 104P. Furthermore, a shallow trench isolation (STI) 116 is formedbetween the readout circuit 120 and the remaining features of the imagesensor device 100. In some embodiments, the STI 116 is formed of amaterial similar to that of the isolation structure 112. In contrast tothe isolation structure 112, the STI 116 is exposed through the secondsurface 101B of the substrate 101. In some embodiments, the STI 116 isformed in the well region 114.

Referring to FIG. 1B, geometries of the image sensor device 100 areshown from a plan view. The readout circuit 120 is disposed next to thefirst photodiode 102. The components of the readout circuit 120, such asthe reset transistor 150, the amplifier transistor 160 and the selecttransistor 170 may be arranged in a row. In some embodiments, the firstlayer 102P of the first photodiode 102 has a rectangular shape, whereone corner of the rectangle is replaced with the gate electrode 140G andanother corner is replaced with the floating diffusion region 126. It isunderstood that other shapes, such as a circular shape or polygonalshape, for the first layer 102P are also possible. In the presentembodiments, each of the floating diffusion region 126 and the gateelectrode 140G has a triangular shape. Further, the floating diffusionregion 126 and the gate electrode 140G are disposed at diagonallyopposite corners of the first layer 102P. The source/drain region 142 ofthe auxiliary transistor 140 has a rectangular shape and is disposedadjacent to the rectangle of the first layer 102P. The electrode gate140G of the auxiliary transistor 140 has a bevel side 140R facing thegate electrode 130G of the transfer transistor 130. In some embodiments,the area of the electrode gate 130G can be enlarged by making the bevelside 140R closer to the horizontal portion 130H of the electrode gate130G without contact therebetween. In that situation, the first layer102P is partially covered by the gate electrode 140G. In someembodiments, the electrode gate 140G covers the geometric center 102C ofthe first layer 102P. In an embodiment, the gate electrode 130Gsubstantially covers the first layer 102P while leaving a distance fromthe horizontal portion 130H of the gate electrode 130G. Furthermore, thedistances W1, W2, W3, D1 and D2 described with reference to FIG. 1A arealso identified in FIG. 1B from a plan view.

FIGS. 1C and 1D are schematic layouts of doped layers 104P and 104N ofthe image sensor device 100, in accordance with some embodiments.Referring to FIG. 1C, the first layer 104P of the second photodiode 104has an area (depicted as the dotted region) greater than that of thefirst layer 102P of the first photodiode 102. In some embodiments, thefirst layer 104P overlaps the readout circuit 120, i.e., the transistors150, 160 and 170. In some embodiments, the second layer 104P partiallyoverlaps the horizontal portion 130H. In some embodiments, the secondlayer 104P is laterally separate from the vertical portion 130V by adistance D3. In some embodiments, a ratio D3/D0 is between about 0.05and about 0.25, wherein the length D0 is a length of one side of thefirst layer 104P. In some embodiments, the distance D3 is between about50 nm and about 250 nm. Referring to FIG. 1D, the second layer 104N(depicted as the dotted region) of the second photodiode 104 has asubstantially rectangular shape. In some embodiments, the second layer104N has an area less than an area of the first layer 104P. Anotherdifference between the second layer 104N and the first layer 104P isthat the second layer 104N extends below the vertical portion 130V ofthe electrode gate 130G, as can also be seen in FIG. 1A.

FIG. 2 is a schematic circuit diagram of the image sensor device 100 inFIG. 1A, in accordance with some embodiments. Both of the firstphotodiode 102 and the second photodiode 104 are coupled to the transfertransistor 130 and the auxiliary transistor 140. In some embodiments,the first photodiode 102 and the second photodiode 104 are illustratedin FIG. 2 as connected in parallel. Although it is shown in FIG. 1A thatthe first photodiode 102 and the second photodiode 104 are cascaded in astack, in operation of electron transfer, the generated charges in eachof the first photodiode 102 and the second photodiode 104 can beindividually transferred to the floating diffusion region 126 inparallel. The image sensor device 100 further includes a variableresistance R1 between the first photodiode 102 and the second photodiode104. The variable resistance R1 represents a path of charge transferbetween the photodiodes 102 and 104 after electrons generated from onephotodiode may be directed to the other photodiode before they aretransferred to the floating diffusion region 126. The variableresistance resulting from the varying current may be dependent upon theamounts of photo-generated charges in a single data sampling operation.Operation details associated with charge transfer between the twophotodiodes 102 and 104 will be provided in subsequent paragraphs.

The floating diffusion region 126 is coupled to a source/drain region(e.g., region 156 in FIG. 1A) of the reset transistor 150 and the gateelectrode 160G of the amplifier transistor 160. The gate electrode 150Gis coupled to a reset signal in a reset operation. Another source/drainregion (e.g., region 158 in FIG. 1A) of the reset transistor 150 iselectrically coupled to a supply voltage VDD1. A source/drain region(e.g., region 166 in FIG. 1A) of the amplifier transistor 160 iselectrically coupled to a supply voltage VDD2, and another source/drainregion (e.g., region 168 in FIG. 1A) of the amplifier transistor 160 iselectrically coupled to the source/drain region (e.g. region 176) of theselect transistor 170. The gate electrode 170G of the select transistor170 is coupled to a row select signal in a reset operation. A conversionvoltage Vout at another source/drain region (e.g., region 178) is fed toan output data line DL. In some embodiments, the converted voltage Voutis provided to an amplifier circuit AMP for further signal processing.In some embodiments, the select transistor 170 is coupled to a currentsource device CS through the data line DL to facilitate voltage samplingin the readout operation.

FIG. 3A is a schematic diagram of potential profiles associated with theimage sensor device of FIG. 1A during a charge integration period, inaccordance with some embodiments. The potential profiles are drawn alongsectional lines BB′ and CC′ in FIG. 1A. Four phases of chargeaccumulation (integration) are shown in FIG. 3A where the y-axisrepresents a potential (voltage) value, in which the arrow of the y-axispoints downward to illustrate the potential of accumulated electronsmore clearly. The sectional line BB′ in FIG. 1A transverses theisolation structure 112 (ISO), the second photodiode 104 (DPD) and firstphotodiode 102 (SPD), the gate electrode 140G (TR) of the auxiliarytransistor 140 and the source/drain region 142 (VA). Similarly, thesectional line CC′ in FIG. 1A transverses the isolation structure 112(ISO), the second photodiode 104 (DPD) and first photodiode 102 (SPD),the gate electrode 130G (VTX) of the transfer transistor 130 and thefloating diffusion region 126 (FD). Through proper arrangements of theabove-mentioned nodes, a potential profile suitable for chargeintegration can be established. For example, the isolation structure 112(ISO) and the gate electrodes 130G (VTX) and 140G (TR) may be biased atvoltages VB0, VB2 and VB3, respectively. In some embodiments, the firstphotodiode 102 (SPD) and the second photodiode 104 (DPD) are fabricatedwith different doping concentrations so as to provide an interface (IF)with a barrier voltage VB1 between the first photodiode 102 (SPD) andthe second photodiode 104 (DPD). In addition, the floating diffusionregion 126 (FD) and source/drain 142 (VA) are biased at relatively highvoltages. The barrier voltages VB1, VB2 and VB3 collaborativelyconstruct a charge tank in the first photodiode 104 (SPD) to keep thegenerated electrons from flowing to adjacent pixels. Similarly, thebarrier voltages VB0 and VB1 collaboratively construct another chargetank in the second photodiode 104 (DPD) to retain the generatedelectrons. In some embodiments, the barrier voltage VB0 is less than thebarrier voltage VB3. In some embodiments, the potential of the secondphotodiode 104 (DPD) is higher than the potential of the firstphotodiode 102 (SPD). In some embodiments, the barrier voltages follow arelationship of VB0<VB3<VB2<VB1.

Still referring to FIG. 3A, assume for the sake of discussion thatelectrons are generated by the second photodiode 104 (DPD). Referring tothe subplot of Phase 1, electrodes are accumulated in the secondphotodiode 104 (DPD) at a beginning stage of charge integration. AtPhase 2, when the accumulated electrons reach a potential less thanbarrier voltage VB1, excess electrons start to flow to the firstphotodiode 102 (SPD). That is because electrons have a negativepolarity; thus more accumulated charges results in a higher electronpotential with less potential value. Subsequently, the second photodiode104 (DPD) continues to receive radiation in Phase 3. The electrons reacha potential that becomes lower than the voltage VB1 of the interfacebarrier IF yet is still equal to or higher than the barrier voltagesVB0, VB2 and VB3. As a result, substantially no free electrons flow outof the first photodiode 102 (SPD) or the second photodiode 104 (DPD).During Phase 4 when the electrons reach an accumulated potential lowerthan the barrier voltage VB2 of the auxiliary transistor 140 (TR) andhigher than the barrier voltage VB3 of the transfer transistor 130,electrons overflow to the source/drain region 142 (VA). In that case,the electrons have reached the well capacity since additional electronscannot be accommodated in either the first photodiode 102 (SPD) or thesecond photodiode 104 (DPD). Since the barrier voltage VB3 is lower thanthe barrier voltage VB2, overflow electrons are drained away through thesource/drain region 142 rather than transferred into the floatingdiffusion region 126 during the course of charge integration. Inaddition, the overflow charges are prevented from entering adjacent unitpixels because they are directed to the source/drain region 142 (VA)instead. The blooming effect can be reduced accordingly. Referring toFIGS. 2 and 3A, the flow of electrons from the second photodiode 104 tothe first photodiode 102 is represented by the variable resistance R1.The flow of electrons through the variable resistance R1 from the firstphotodiode 102 to the second photodiode 104 is described with referenceto FIG. 3B, as provided below.

FIG. 3B is a schematic diagram of potential profiles associated with theimage sensor device of FIG. 1A during a charge integration period, inaccordance with some embodiments. In contrast to FIG. 3A, it is assumedin FIG. 3B that the charges are generated mainly by the first photodiode102 (SPD). The process of charge integration and accumulation in thefirst photodiode 102 (SPD) and the second photodiode 104 (DPD) shown inFIG. 3B is similar to that in FIG. 3A except that the electrons firstreach the well capacity of first photodiode 102 (SPD), followed by anelectron flow from the first photodiode 102 (SPD) toward the secondphotodiode 104 (DPD).

FIG. 4A is a timing diagram of a readout operation of the image sensordevice in FIG. 1A, in accordance with some embodiments. The voltagesettings for the relevant nodes are shown in Table I.

TABLE I Voltage setting for operating an image sensor device Node StatusValue Select transistor V 1-H Supply voltage (e.g., 2.8 V) 170 (SEL) V1-L Ground voltage (e.g., −1.0 V-0 V) Reset transistor V 2-H Supplyvoltage (e.g., 2.8 V) 150 (RST) V 2-L Ground voltage (e.g., −1.0 V-0 V)Transfer transistor V 3-H Supply voltage (e.g., 2.8 V) 130 (VTX) V 3-L−2.0 V < V 3-L < 0 V(e.g., −1.2 V) Auxiliary transistor V 4-H −1.2 V <(V 4-H) < 0 V (e.g., −0.5 V) 140 (TR) V 4-L −2.0 V < V 4-L < 0 V (e.g.,−1.2 V), V 4-L < V 4-H Source/drain V 5-H Supply voltage (e.g., 2.8 V)region 142 (VA) V 5-L 0.5 V < V 5-L < V 5-H, (e.g., 1.0 V)

Before the start of the readout operation, the select transistor 170(SEL), reset transistor 150 (RST) and the transfer transistor 130 (VTX)are turned off through biasing voltages V1-L, V2-L and V3-L,respectively. During operation, the select transistor 170 (SEL) isturned on by a pulse starting at time instant t0 through a biasingvoltage V1-H until the end of a readout period P11. When the resettransistor 150 (RST) receives a reset voltage V2-H at time instant t1,the reset transistor 150 (RST) is turned on, causing the floatingdiffusion region 126 (FD) to reset, thereby draining electrons away andsetting the potential of the doping region 126 to a predeterminedvoltage, such as the supply voltage. The reset is performed for a periodP12 until time instant t2. When the transfer transistor 130 (VTX)receives an enable voltage V3-H at time instant t4, charges aretransferred to the floating diffusion region 126 by an electric fieldgenerated by the gate electrode 130G. The charge transfer is performedfor a pulse period P13 until time instant t5. When the transfertransistor 130 (VTX) returns to an “off” state at time instant t5,charge transfer is terminated. In some embodiments, the pulse period forthe reset transistor 150 (RST) is between about 200 μs and about 2000μs. In some embodiments, the pulse period for the transfer transistor130 (VTX) is between about 200 μs and about 2000 μs.

The auxiliary transistor 140 (TR) is employed to generate an additionalelectric field that magnifies the electric field contributed by thetransfer transistor 130 (VTX). Moreover, the pulse for enabling theauxiliary transistor 140 may be substantially synchronous with the pulsefor enabling the transfer transistor 130 (VTX). Initially, the auxiliarytransistor 140 (TR) is disabled and set at the voltage V4-L. At timeinstant t3, the auxiliary transistor 140 (TR) is turned on through apulse with period P14 that biases the electrode gate 140G at a voltageof V4-H. The auxiliary transistor 140 (TR) may be turned off at timeinstant t5. In some embodiments, time instant t3 is aligned with timeinstant t4. In some embodiments, time instant t3 is ahead of timeinstant t4 by a length of, for example, between about 50 ns and 500 ns.In some embodiments, the auxiliary transistor 140 (TR) is turned off ata time behind time instant t5 by a length of, for example, between about20 ns and 200 ns.

In some embodiments, the source/drain region 142 (VA) is biased with apulse, with voltage V5-L and period P15, substantially synchronous withthe pulse for the auxiliary transistor 140 (TR). The pulse for biasingthe source/drain region 142 (VA) further generates an additionalelectric field that magnifies the electric field generated by thetransfer transistor 130 (VTX) and the auxiliary transistor 140 (TR). Insome embodiments, the pulse period P15 is substantially equal to thepulse period P14. In some embodiments, the start time of period P15 isahead of time instant t3 by a length of, for example, between about 50ns and 500 ns. In some embodiments, the end time of period P15 is behindtime instant t5 by a length of, for example, between about 20 ns and 200ns. Since the source/drain region 142 (VA) is biased at the supplyvoltage when it is deactivated, a coupled capacitive effect may occurbetween the gate electrode 140G and the source/drain region 142. Thecapacitive effect is reduced through biasing the drain/source region 142(VA) at the voltage V5-L.

FIG. 4B is a schematic potential profile associated with the readoutoperation in FIG. 4A, in accordance with some embodiments. The potentialprofile is plotted along a straight sectional line running through theregions 142 and 126 and connecting points B′ and C′ in FIG. 1A. Thedotted line, dashed line and solid line illustrated in FIG. 4Brespectively represent the potential variation under the influence ofthe auxiliary transistor 140 and the source/drain region 142. Theapplication of an additional negative potential, e.g., −1.2V, exertedthrough the electrode gate 140G, magnifies the potential gradientbetween the ends B′ (electrode gate 140G) and C′ (floating diffusionregion 126). Further, the application of another potential, e.g., 1.0V,exerted through the source/drain region 142, can aid in increasing theelectric field gradient for charge transfer. The readout speed of imagedata can be thus increased thereby reducing the image lag effect.

FIGS. 4C and 4D are schematic potential diagrams of the first photodiode102 associated with the readout operation in FIG. 4A, in accordance withsome embodiments. Similar labels for each node used in FIG. 3A, such asVA, TR, SPD. VTX, etc., are reused in denoting same features. Exemplarybiased values for the nodes are illustrated beside the respective node.Referring to FIG. 4C, a potential profile around the first photodiode102 (SPD) before data readout is performed (corresponding to any timeprior to time instant t4). The charges are retained in the firstphotodiode 102 (SPD) without overflowing out. Subsequently, at timeinstant t4, through appropriate bias configuration (exemplified in TableI and FIG. 4B), an enhanced electric gradient between the auxiliarytransistor 140 (TR) and the floating diffusion region 126 (FD) isgenerated, thereby facilitating charge transfer from the firstphotodiode 102 (SPD) to the floating diffusion region 126 (FD).

FIGS. 4E and 4F are schematic potential diagrams of the secondphotodiode 104 associated with the readout operation in FIG. 4A, inaccordance with some embodiments. Referring to FIG. 4E, a potentialprofile around the second photodiode 104 (DPD) before data readout isperformed (corresponding to any time prior to time instant t4). At timeinstant t4, through appropriate bias configuration (exemplified in TableI and FIG. 4B), the electric field between the auxiliary transistor 140(TR) and the floating diffusion region 126 (FD) generated for the firstphotodiode 102 (SPD) can also be utilized for the second photodiode 104(DPD), thereby facilitating charge transfer from the second photodiode104 (DPD) to the floating diffusion region 126 (FD).

FIG. 5A is a timing diagram of a shutter operation of an image sensordevice, in accordance with some embodiments. The voltage settings forthe nodes are shown in Table I. The select transistor 170 (SEL) and thesource/drain region 142 (VR) are kept as V1-L and V5-H, respectively, toremain in an “off” state in the shutter operation. During operation,when the reset transistor 150 (RST) receives a reset voltage V2-1H attime instant t0, the reset transistor 150 (RST) is turned on for a pulseperiod P21 until time instant t5. When the transfer transistor 130 (VTX)receives a pulse with a voltage V3-H at time instant t1, electrons aredrained away through the reset transistor 150 (RST) by an electric fieldgenerated by the gate electrode 130G. The charge reset is performed fora pulse with a period P22 until time instant t2. In some embodiments,the pulse period P22 for the transfer transistor 130 (VTX) is betweenabout 200 μs and about 2000 μs.

The auxiliary transistor 140 (TR) is configured to generate anadditional electric field that aids in resetting the charges in thephotodiodes 102 and 104. Moreover, the biasing pulse for the auxiliarytransistor 140 is offset from the biasing pulse for the transfertransistor 130 (VTX). Initially, the auxiliary transistor 140 (TR) isdisabled and set at the voltage V4-L. At time instant t3, the auxiliarytransistor 140 (TR) is turned on through a pulse with a period P23 andvoltage V4-H. The auxiliary transistor 140 (TR) may be turned off attime instant t4 prior to the time instant t5. In some embodiments, timeinstant t3 is behind time instant t2 by a length of, for example,between about 20 ns and 200 ns. In some embodiments, time instant t4 isahead of time instant t5 by a length of, for example, between about 20ns and 200 ns. In some embodiments, the pulses with periods P22 and P23are arranged in a substantially synchronous manner. In some embodiments,the occurrences of two pulses can be interchanged, i.e., the pulse withperiod P23 for enabling the auxiliary transistor 140 (TR) can bedisposed prior to the pulse with period P22 for enabling the transfertransistor 130 (VTX). In some embodiments, the pulses P22 and P23 arenot overlapped with each other.

FIGS. 5B, 5C and 5D are schematic potential diagrams of the photodiodes102 and 104 associated with the shutter operation in FIG. 5A, inaccordance with some embodiments. Similar labels for nodes in FIG. 4A,such as VA, TR, SPD, DPD, and VTX, are reused for denoting samefeatures. Label RSTD represents a bias voltage, e.g., a supply voltagelevel, applied to the source/drain region 158 of the reset transistor150 (RST). Referring to FIG. 5B, a potential profile for the firstphotodiode 102 (SPD), the second photodiode 104 (DPD) and neighboringnodes thereof prior to the shutter operation is shown. In cases wherecharges generated in a previous image data sampling are not read outcompletely, some residual charges may be left in the first photodiode102 (SPD), the second photodiode 104 (DPD) or the floating diffusionregion 126 (FD). Image data detection accuracy is thus deteriorated. Asillustrated in FIG. 5C, at time instant t1 of FIG. 5A, throughappropriate bias configuration (exemplified in Table I and FIG. 5A), afirst electric field is generated through the transfer transistor 130(VTX) that drains all residual charges in the second photodiode 104(DPD) and the floating diffusion region 126 (FD), and at least a portionof the residual charges in the first photodiode 102 (SPD). Subsequently,at time instant t3, a second electric field is generated by theauxiliary transistor 140 (TR) and assists in removing any charges thatare not drained by the first electric field. The first electric fieldand the second electric field are generated to drain charges alongsubstantially opposite directions toward the transfer transistor 130 andthe auxiliary transistor 140, respectively. Residual charges, especiallythose left in the first photodiode 102 (SPD), can be cleaned by help ofthe two electric fields.

FIGS. 6A and 6B are a schematic circuit diagram and a plan view,respectively, of an image sensor device 600, in accordance with someembodiments. Referring to FIG. 6A, the image sensor device 600 includesa pixel array arranged in rows and columns where four exemplary unitpixels 610A through 610D are illustrated. Each of the four unit pixelshas a transfer transistor 130, first photodiode 102, second photodiode104 and auxiliary transistor 140 along with the source/drain region 142.In the depicted embodiment, the four unit pixels share one floatingdiffusion region 126. The readout circuit 120 is electrically coupled tothe pixel array through the floating diffusion region 126. Referring toFIGS. 1B and 6B, the floating diffusion region 126 is disposed at acenter of the array formed by the four unit pixels 610A through 610D. Insome embodiments, the floating diffusion region 126 has a polygonalshape, such as an octagonal shape, a rectangular shape or a squareshape. Moreover, the isolation structure 112 separates the unit pixelsby filling spaces among the four unit pixels 610A through 610D.

FIGS. 6C and 6D are schematic layouts of doped layers 104P and 104N ofthe second photodiode 104 in the image sensor device 600, in accordancewith some embodiments. Referring to FIGS. 1C and 6C, the first layers104P of adjacent unit pixels are connected to each other, and aredepicted as the whole dotted region in FIG. 6C. In some embodiments, thefirst layer 104P is separate from the vertical portion 130V by adistance D3 in a configuration similar to those in FIG. 1C. In someembodiments, at least one first layer 104P extends to overlap thereadout circuit 120. i.e., the transistors 150, 160 and 170. In someembodiments, a ratio D3/D0 is between about 0.05 and about 0.25, whereinthe length D0 is a length of one side of the first layer 104P. In someembodiments, the first layers 104P of the four unit pixels form a ringsurrounding the floating diffusion region 126. In some embodiments, thering of the first layers 104P is separate from the floating diffusionregion 126. Referring to FIG. 6D, the second layers 104N of the fourunit pixels are separated from each other and are depicted as fourdisjoined dotted regions. Each second layer 104N has a polygonal shape,such as a rectangular shape. In some embodiments, each second layer 104Nextends below the respective horizontal portion 130H and the verticalportion 130V. In some embodiments, each second layer 104N of the fourunit pixels fully overlaps a respective electrode gate 130G of thetransfer transistor 130, including the respective horizontal portion13011 and the vertical portion 130V. In some embodiments, each secondlayer 104N of the four unit pixels at least partially overlaps thefloating diffusion region 126.

In accordance with some embodiments of the present disclosure, an imagesensor semiconductor device includes a semiconductor substrate and afirst photodiode disposed in the semiconductor substrate and configuredto generate charges in response to radiation. The image sensorsemiconductor device also includes a first transistor disposed adjacentto the first photodiode, and a second transistor disposed over the firstphotodiode, wherein the first transistor and the second transistor areconfigured to generate at least one electric field to move the chargesgenerated by the first photodiode. The image sensor device furtherincludes a floating diffusion region configured to store the movedcharges.

In accordance with some embodiments of the present disclosure, an imagesensor semiconductor device includes a semiconductor substrate and aplurality of unit pixels. Each of the plurality of unit pixels comprisesa first photodiode disposed in the semiconductor substrate andconfigured to generate charges in response to radiation, a firsttransistor disposed over the first photodiode, and a second transistordisposed over the first photodiode. At least one of the first transistorand the second transistor is configured to generate an electric fieldmoving charges generated by the first photodiode.

In accordance with some embodiments of the present disclosure, a methodincludes configuring a photodiode to generate charges in response toradiation; biasing a first transistor to generate a first electric fieldthat transfers a first portion of the charges from the photodiode to afloating diffusion region in a first period; and biasing a secondtransistor to generate a second electric field that transfers a secondportion of the charges to the floating diffusion region or asource/drain region.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An image sensor device, comprising: asemiconductor substrate; a first photodiode disposed in thesemiconductor substrate and configured to generate charges in responseto radiation; a first transistor disposed adjacent to the firstphotodiode; a floating diffusion region configured to store thegenerated charges; a reset transistor configured to reset the floatingdiffusion region; and a second transistor disposed over the firstphotodiode, and the second transistor also disposed between the resettransistor and the floating diffusion region, wherein the firsttransistor and the second transistor are configured to generate a firstelectric field and a second electric field, respectively, to move thecharges generated by the first photodiode to the floating diffusionregion, wherein the first transistor and the second transistor areenabled by a first pulse and a second pulse, respectively, during ashutter operation, wherein the first pulse and the second pulse have asame polarity, wherein the first transistor and the second transistorare further enabled by a third pulse and a fourth pulse, respectively,during a readout operation for providing a voltage in response to apotential at the floating diffusion region, and the third pulse and thefourth pulse have opposite polarities, wherein the second transistor andthe floating diffusion region are disposed on opposite sides of thefirst photodiode.
 2. The image sensor device according to claim 1,wherein the second transistor is further configured to generate a thirdelectric field to move the charges to a source/drain region of thesecond transistor.
 3. The image sensor device according to claim 1,further comprising an amplifier transistor and a readout transistorconfigured to provide a voltage in response to the charges stored in thefloating diffusion region.
 4. The image sensor device according to claim1, wherein the first transistor and the second transistor are disposedon diagonally opposite sides of the first photodiode.
 5. The imagesensor device according to claim 1, wherein the first pulse is offsetfrom the second pulse.
 6. The image sensor device according to claim 1,wherein the third pulse is synchronous with the fourth pulse during thereadout operation.
 7. The image sensor device according to claim 1,wherein the second transistor comprises a source region or a drainregion configured to generate the second electric field during thereadout operation.
 8. The image sensor device according to claim 1,further comprising a second photodiode in the semiconductor substrate,wherein the first photodiode and the second photodiode are stacked in adepth direction of the semiconductor substrate.
 9. The image sensordevice according to claim 8, further comprising a channel in thesemiconductor substrate, wherein each of the first photodiode and thesecond photodiode comprises a first layer having a first dopant type anda second layer having a second dopant type, and the channel issurrounded by the first photodiode, the second photodiode and the firsttransistor.
 10. The image sensor device according to claim 9, furthercomprising an isolation structure laterally surrounding the second layerof the second photodiode.
 11. The image sensor device according to claim8, wherein the second transistor at least partially overlaps the secondphotodiode vertically.
 12. An image sensor device, comprising: asemiconductor substrate; a plurality of unit pixels; a floatingdiffusion region disposed at a center of the plurality of unit pixels;and a reset transistor adjacent to the plurality of unit pixels andconfigured to reset the floating diffusion region;, wherein each of theplurality of unit pixels comprises: a first photodiode disposed in thesemiconductor substrate and configured to generate first charges inresponse to radiation; a second photodiode configured to generate secondcharges in response to the radiation; a first transistor disposed overthe first photodiode; and a second transistor disposed over the firstphotodiode, wherein the second transistor is also disposed between thereset transistor and the floating diffusion region, the first transistorand the second transistor being configured to generate an electric fieldmoving the first charges generated by the first photodiode to thefloating diffusion, wherein the floating diffusion region is configuredto store the first and second charges generated by the plurality of unitpixels, and wherein the first transistor and the second transistor areenabled by a first pulse and a second pulse, respectively, during ashutter operation, wherein the first pulse and the second pulse have asame polarity, wherein the first transistor and the second transistorare further enabled by a third pulse and a fourth pulse, respectively,during a readout operation for providing a voltage in response to apotential at the floating diffusion region, wherein the third pulse andthe fourth pulse have opposite polarities.
 13. The image sensor deviceaccording to claim 12, wherein the floating diffusion region is furtherconfigured to receive the second charges generated by each of the secondphotodiodes.
 14. The image sensor device according to claim 13, whereineach of the second photodiodes comprises a first layer having a firstdopant type and a second layer having a second dopant type, and thefirst layers of the second photodiodes in adjacent unit pixels arejoined to each other.
 15. A method of operating an image sensor device,the method comprising: configuring a photodiode to generate charges inresponse to radiation; biasing a first transistor by a first pulse togenerate a first electric field that transfers a first portion of thecharges from the photodiode to a floating diffusion region in a firstperiod during a shutter operation; and biasing a second transistor by asecond pulse to generate a second electric field that transfers a secondportion of the charges from the photodiode to a source or a drain regionof the second transistor during the shutter operation, wherein thesecond transistor and the floating diffusion region are disposed onopposite sides of the photodiode, and the second transistor is disposedover the first photodiode and also is disposed between the resettransistor and the floating diffusion region, wherein the first pulseand the second pulse have a same polarity, wherein the first transistorand the second transistor are further enabled by a third pulse and afourth pulse, respectively, during a readout operation for providing avoltage in response to a potential at the floating diffusion region, andthe third pulse and the fourth pulse have opposite polarities.
 16. Themethod according to claim 15, wherein the third pulse is synchronouswith the fourth pulse during the readout operation.
 17. The methodaccording to claim 15, wherein the first pulse is offset from the secondpulse during the shutter operation.